1. Field of the Invention
This invention relates generally to fuel cell systems, and more particularly, to techniques for managing fluid flow throughout the fuel cell system.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity from fuel and oxygen. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials in liquid form, such as methanol are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before the hydrogen is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external fuel processing. Many currently available fuel cells are reformer-based. However, because fuel processing is complex and generally requires costly components which occupy significant volume, reformer based systems are more suitable for comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as for somewhat larger scale applications. In direct oxidation fuel cells of interest here, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is directly introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is the direct methanol fuel cell or DMFC system. In a DMFC system, a mixture comprised of predominantly methanol or methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidant. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate, as is discussed further hereinafter.
Typical DMFC systems include a fuel source or reservoir, fluid and effluent management systems, and air management systems, as well as the direct methanol fuel cell (“fuel cell”) itself. As used herein, the term “fuel cell system” shall include systems that include a single fuel cell, multiple fuel cells coupled in a fuel cell array, and/or a fuel cell stack. The fuel cell typically consists of a housing, hardware for current collection, fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system take place at and within the MEA. In the fuel oxidation process at the anode, the fuel typically reacts with water and the products are protons, electrons and carbon dioxide. Protons from hydrogen in the fuel and in water molecules involved in the anodic reaction migrate through the proton conducting membrane electrolyte (“PCM”), which is non-conductive to the electrons. The electrons travel through an external circuit, which contains the load, and are united with the protons and oxygen molecules in the cathodic reaction. The electronic current through the load provides the electric power from the fuel cell. The invention set forth herein can also be implemented with any fuel cell system with a single pump and multiple valves for managing fluids within a fuel cell system including direct oxidation fuel cell systems and reformer-based systems. The invention can be implemented in fuel cell systems that use a proton exchange medium other than as described herein including but not limited to those systems that implement a silicon or liquid electrolyte.
A typical MEA includes an anode catalyst layer and a cathode catalyst layer sandwiching a centrally disposed PCM. One example of a commercially available PCM is NAFION® (NAFION® is a registered trademark of E.I. Dupont de Nemours and Company), a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. A PCM that is optimal for fuel cell applications possesses a good protonic conductivity and is well-hydrated. On either face of the catalyst coated PCM, the MEA further typically includes a “diffusion layer”. The diffusion layer on the anode side is employed to evenly distribute the liquid or gaseous fuel over the catalyzed anode face of the PCM, while allowing the reaction products, typically gaseous carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen to the cathode face of the PCM, while minimizing or eliminating the accumulation of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM to the current collector.
Direct oxidation fuel cell systems for portable electronic devices ideally are as small as possible for a given electrical power and energy requirement. The power output is governed by the rates of the reactions that occur at the anode and the cathode of the fuel cell operated at a given cell voltage. More specifically, the anode process in direct methanol fuel cells, which use acid electrolyte membranes including polyperflourosulfonic acid and other polymeric electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, water molecules are consumed to complete the oxidation of methanol to a final CO2 product in a six-electron process, according to the following electrochemical equation:CH3OH+H2OCO2+6H++6e−  1)
Generally, in order to maintain process (1) during fuel cell operation, it is important that fluid flow throughout the fuel cell system is balanced correctly. More specifically, the delivery of fuel at the appropriate concentration is a consideration and it varies with fuel cell operating conditions and ambient conditions. Secondly, water management is an important consideration because water is a reactant in the anodic process at a molecular ratio of 1:1 (water:methanol), so that the supply of water, together with methanol to the anode at an appropriate weight (or volume) ratio is critical for sustaining this process in the fuel cell system. In addition, water is generated at the cathode, and this cathode-generated water can be recirculated to the anode for use in the anodic portion of the process (1). Water is also important for maintaining adequate hydration of the membrane. However, too much water can lead to cathode flooding. Thus, it is desirable to finely control the water balance throughout the fuel cell system.
The present invention is described in conjunction with a stack comprised of more than one fuel cell, and which typically include more than one bipolar plate. However, those skilled in the art will recognize that the precise configuration of the fuel cells may comprise a single fuel cell, or a plurality of fuel cells arranged in a substantially planar system, while remaining within the scope of the present invention.
Some systems that have water management techniques have been known such as active systems which are based on feeding the cell anode with a very diluted (2%) methanol solution, pumping excess amounts of water at the cell cathode back to cell anode and dosing the recirculation liquid with neat methanol stored in a reservoir. Such active systems that include pumping can provide, in principle, maintenance of appropriate water level in the anode by dosing the methanol from a fuel delivery cartridge into a recirculation loop. The loop also receives water that is collected at the cathode and pumped back into the recirculation anode liquid. In this way, a desired water/methanol anode mix can be maintained. However, the multiple pumps that are needed to carry the various solutions throughout the fuel cell can lead to parasitic losses that ultimately result in a less efficiently operating fuel cell system. This has been particularly true in high power applications in which a fuel cell stack is employed.
Another challenge arises in a system containing a fuel cell stack when it is necessary to purge the stack of fluids. This procedure might be performed to change the fuel concentration if a lower or higher than desired concentration has developed within the stack. Other situations in which a stack purge is performed is when the system is to be shutdown for a routine maintenance check or for repairs, where the pressure within the fuel cell is greater than desired, or where it is desirable to put the fuel cell stack in a freeze tolerant state.
Temperature regulation is also a consideration in fuel cell system management. For example, fuel cell operating temperatures must be regulated so that the build up of excess heat is controlled. Sometimes excess heat must be dissipated. Ambient environmental conditions are also a factor in the dissipation of heat, and affect fuel cell performance, particularly in sub-freezing ambient environments.
Based upon all of these considerations, there remains a need for controlling the flow of fluids and controlling temperature in a fuel cell system, and specifically, there is a need for a fuel cell system in which the flow of fuel, water, effluents and other gases can be finely controlled depending upon the desired operating characteristics of the fuel cell system or the ambient environmental conditions. There remains a further need for a system that incorporates this functionality, but that does not require multiple pumps, even when the fuel cell system operates using a fuel cell stack for high power applications.